This invention relates to the field of storing spent nuclear fuel and specifically to a ventilated vertical overpack for storing spent nuclear fuel.
In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, spent nuclear fuel is first placed in a canister. The loaded canister is then transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel for a determined period of time.
One type of storage cask is a ventilated vertical overpack (xe2x80x9cVVOxe2x80x9d). A VVO is a massive structure made principally from steel and concrete and is used to store a canister loaded with spent nuclear fuel. Typically, VVOs are cylindrical in shape and are extremely heavy, weighing over 150 tons and often having a height greater than 15 feet. VVOs have a flat bottom, a cylindrical body having a chamber adapted to receive a canister of spent nuclear fuel, and a removable top lid.
In using a VVO to store spent nuclear fuel, a canister loaded with spent nuclear fuel is placed in the chamber of the cylindrical body of the VVO. Because the spent nuclear fuel is still producing a considerable amount of heat when it is placed in the VVO for storage, it is necessary that this heat energy have a means to escape from the VVO chamber. This heat energy is removed from the outside surface of the canister by ventilating the VVO chamber. In ventilating the VVO chamber, cool air enters the VVO chamber through bottom ventilation ducts, flows upward past the loaded canister, and exits the VVO at an elevated temperature through top ventilation ducts. The bottom and top ventilation ducts are located circumferrentially near the bottom and top of the VVO""s cylindrical body respectively.
Because it is imperative that the canister of spent nuclear fuel not be directly exposed to the external environment, the chamber has a pedestal situated at its bottom. When the canister is placed in the chamber for storage, the canister rests on the pedestal, ensuring that the canister is located at an elevation well above the openings of the bottom ventilation ducts. Additionally, the top ventilation ducts are positioned on the cylindrical body so that the openings are located well above the canister when the canister is resting on the pedestal inside the VVO chamber. Because the canister is not directly exposed to the external environment, the extent of radiation emanating through the ducts to the external environment is negligible. However, positioning the bottom ventilation ducts on the VVO body so that they are below the canister and positioning the top ventilation ducts on the VVO body so that they are above the canister results in an increased length of the VVO body. This increased length can result in the VVO being too tall to complete canister transfer operations inside a nuclear power plant building because the VVO will not fit through the door. As such, extra money must be spent either to construct an autonomous external canister transfer facility, or to enlarge the door of the power plant.
In most nuclear power plants, a canister loaded with spent nuclear fuel is transferred from a radiation pool to a VVO by a transfer cask. In transferring the loaded canister from the transfer cask to the VVO, the transfer cask is stacked atop a VVO with its lid removed so that the canister can be lowered into the VVO""s chamber. During the lowering operation, the canister must pass through the elevation where the top ventilation ducts are located, creating a direct path for radiation escape. Because it is undesirable to directly expose a loaded canister of spent nuclear to the environment at any time, the openings of the top ventilation ducts on the VVO body must be closed during transfer. This is done by installing temporary shield plugs in the openings. Because these temporary shield plugs must provide ample radiation blockage, they are made of concrete and are often massive, posing logistical problems in the handling effort needed to install and remove them in the top ventilation ducts that are located at least 15 feet above the floor. For example, installing and removing four shield blocks entails eight heavy load-handling evolutions which increases the potential of a load drop mishap (and operator injury). Additionally, removal of the shield blocks following canister transfer operations increases the radiation dose exposure to the operations personnel.
Moreover, the potential for leakage of radiation through the top ventilation ducts during the lowering of the loaded canister into the VVO chamber is quite real, even with the shield plugs installed in the top ventilation ducts, because of the narrow crevice that must exist between the shield plugs and the top ventilation duct openings for tolerance reasons.
These problems and other are solved by the present invention which in one aspect is a lid for a ventilated vertical overpack having a chamber for receiving spent nuclear fuel, the lid having ventilation means. The lid can comprise a lid cap and a lid body wherein the ventilation means are located on the lid body.
Preferably, the ventilation means are one or more lid ventilation ducts. Also preferably, the lid comprises a lid shear ring and has means to secure the lid to the ventilated vertical overpack body.
In another aspect, the invention is a system for storing spent nuclear fuel comprising the lid as described above and a ventilated vertical overpack having a cylindrical body including lower ventilation ducts, a bottom, and a chamber formed by the body and the bottom adapted for receiving a canister of spent nuclear fuel.
Preferably, the lower ventilation ducts are located on the cylindrical body near the bottom and there are no upper ventilation ducts included on the cylindrical body of the overpack. Also preferably, the lid is secured to the overpack by bolts that extend through the lid and threadily engage the overpack body.
The system""s lid can comprise a lid cap which substantially encloses the chamber and a lid body in which the upper ventilation means are located. The ventilation means in the lid of the system are preferably one or more lid ventilation ducts. Preferably, the system""s lid has a lid shear ring and the cylindrical body has a top surface having a body shear ring, wherein the lid shear ring engages the body shear ring when the lid is placed on the cylindrical body, restricting lateral movement of the lid with respect to the cylindrical body.
Also preferably, when the canister of spent nuclear fuel is received in the overpack and the lid is secured, air within the chamber is warmed by heat from the spent nuclear fuel, cold air entering through the lower ventilation ducts and warmed air exiting through the ventilation means of the lid.
In yet another aspect, the invention is a method of storing spent nuclear fuel comprising placing a canister of spent nuclear fuel in the chamber of the overpack of the system described above; and securing the lid so that air within the chamber is warmed by heat from the spent nuclear fuel, cold air entering through the lower ventilation ducts and warmed air exiting through the ventilation means of the lid. Preferably, the lid is secured to the body of the overpack by bolting it thereto. Also preferably in the method, the lid has a lid shear ring and the cylindrical body has a top surface having a body shear ring, wherein lateral movement of the lid with respect to the cylindrical body is restricted when the lid is placed on the cylindrical body.